This invention relates to organopolysiloxane compositions having improved flame retardant properties. More particularly, this invention is concerned with a composition of matter which, in the cured state, exhibits improved flame retardant properties and which comprises, (1) an organopolysiloxane gum convertible to the cured, solid, elastic state and consisting essentially of silicon atoms, oxygen atoms and organic groups selected from the class consisting of methyl radicals, aromatic radicals selected from the class consisting of aryl and halogenated aryl radicals, vinyl radicals, lower alkyl radicals, lower cyanoalkyl radicals and lower haloalkyl radicals, (2) a finely divided inorganic filler, (3) a platinum compound or platinum and (4) and various effective amounts of hydrated alumina, fumed titanium dioxide, and magnesium oxide, in combination.
A method for improving the flame retardancy of a silicone rubber is shown in U.S. Pat. No. 3,514,424, Nobel et al, where a platinum compound or platinum is combined with other components of a silicone rubber to impart flame retardancy. While the addition of platinum or platinum compound does improve the flame retardancy of silicone rubbers and is entirely adequate for most uses, the rubbers produced according to the Noble et al Patent are not as flame retardant as might be desired.
U.S. Pat. No. 3,734,877, Christie, discloses a flame retardant composition consisting essentially of a silicone elastomer and an appropriate filler in combination with effective amounts of triphenyl phosphite. It has been found that the addition of triphenyl phosphite is not required for the silicone compositions of the present invention.
Other flame resistant compositions are disclosed in U.S. Pat. No. 3,635,874--Laur, and U.S. Pat. No. 3,652,488--Harder. Both patents disclose a silicone elastomer stock as well as inorganic filler, platinum and carbon black. The Laur composition further contains 0.5 to 100 parts by weight of fume titanium dioxide.
The composition of the present invention is superior to those disclosed above in that it does not require the addition of such pigments as carbon black and fume titanium dioxide in order to provide outstanding flame retardant characteristics, especially when measured by the rigorous 60 second Burn test. Of course, the flame retardant properties of these pigments can be utilized if desired.
To the extent that the above-identified patents disclose compositions and methods of compounding silicone elastomers, especially those having flame resistant characteristics, they are hereby incorporated by reference.
The silicone compositions of the present invention are particularly useful for forming silicone rubber products, which meet or surpass Federal Aviation Regulations for flame retardancy when such silicone rubber products are utilized in aircraft. As is well-known, aircraft materials must meet rigorous flamability testing procedures before they can be incorporated into the aircraft. Various tests are utilized to demonstrate the compliance of aircraft materials with FAA regulations for flamability characteristics and often the air worthiness certification of an aircraft can depend in great part upon the ability of raw materials to meet these rigorous standards.
Accordingly, there has been developed what will be called a 60-Second Burn Test which demonstrates the superior flame retardant characteristics of the silicone rubber compositions of the present invention. Specimen dimensions are 3.times.12.times.0.075.+-.0.005 inches. A Bunsen or Tirrill gas burner having an inside diameter tube of approximately, 0.375 inches is utilized. The gas for the ignition source has an approximate composition of 55% hydrogen, 24% methane, 3% ethane and 18% carbon monoxide and has an approximate specific gravity of 0.365 (air=1) and an approximate BTU content of 540 per cubic ft. at 21.degree. C. Test specimens are conditioned prior to testing by exposure to an atmosphere of approximately 70.degree. F. and a relative humidity of 50% for a minimum of 24 hours.
The 60-Second Burn Test takes place in a draft-free cabinet wherein the lighted gas burner is adjusted to a flame height of approximately 1.5 inches. The temperature of the gas flame is measured by a thermocouple at approximately 0.75 in. above the burner orifice. A temperature between 1500.degree. and 2000.degree. F. is preferred. The specimen to be tested is mounted vertically and its bottom edge is flush with the bottom edge of the holding clamp. A timer is started immediately when the burner is brought into position below the sample at a point approximately 0.75 in. above the end of the burner orifice. The flame is held directly beneath this point for 60.+-.0.5 seconds whereupon it is withdrawn. Timing is continued until the flame on the specimen goes out. The specimen should be considered "self-extinguished" if the flame goes out before reaching the top of the specimen. The term "self extinguishing time" will therefore be the recording time minus the 60-second ignition period.
It is therefore an object of the present invention to provide a curable silicone composition having superior flame resistant characteristics.
It is another object to provide a composition which surpasses the rigorous 60-Second Burn Test.
It is another object to provide a composition having flame resistant characteristics yet does not require the use of a carbon black pigment.
It is a further object to provide methods for producing such flame resistant curable silicone compositions.